This invention relates to an apparatus for sensing ionic current flowing between the electrodes of a spark plug in an internal combustion engine.
In a spark ignited internal combustion engine, the spark generated by a spark plug at the time of ignition produces ionization of the air in the cylinder. If a voltage is applied between the electrodes of the spark plug when ions are present, an ionic current is generated between the electrodes. By measuring the ionic current, it is possible to determine whether the cylinder in which the spark plug is disposed misfired based on the magnitude of the ionic current. Furthermore, the magnitude of the ionic current during the combustion stroke of a cylinder is highest when the pressure in the cylinder reaches a maximum, so the ionic current can be used to monitor pressure variations within a cylinder.
Conventional ionic current sensing devices have a power supply connected to a spark plug so as to apply a negative voltage between the center electrode and the ground electrode of the spark plug to produce an ionic current in the form of positive ions. An electric current flowing due to the ionic current through a current sensing resistor connected in series with the power supply is then measured as an indication of the ionic current. However, since the positive ions generating the ionic current have a large mass and a low velocity, the magnitude of the electric current flowing through the current sensing resistor is extremely small and difficult to measure.